The field of the invention relates generally to quantitative or probabilistic risk assessment, and more specifically, to systems and methods for quantitative or probabilistic risk-based design and maintenance. For abbreviation, in the following paragraphs, risk assessment will be used to represent quantitative or probabilistic risk assessment.
Development of advanced risk assessment methodologies has been ongoing for some time. The following example relates to one application where a risk assessment may provide an alternative to physical inspections.
In this particular example, results from a deterministic crack growth analyses of the wing carry through lower cover attachments for a particular aircraft, indicated an immediate inspection was needed. However, there are no credible nondestructive inspection techniques available for inspecting the lower cover attachments as these covers are attached using fasteners. In other words, the attachment area of the cover cannot be inspected without removal of the fasteners. Removal of the fasteners is costly and there is a risk that the holes in the lower cover attachments could be compromised. The structure that underlies the lower cover could also be damaged when the fasteners are being removed. Therefore, for this particular “real world” example, a decision was made to delay the costly and potentially damaging inspections until a credible non-destructive inspection was developed. This decision was based on results of hole testing showed damage tolerance life improvements for this TaperLok hole was on the order of 9 to 10 times that of other hole types.
Unfortunately, no credible crack growth curve had been developed for the TaperLok hole to help extend its life. A set of standards has been established that defines the inspection and repair schedule based on deterministic analyses. However, there is no information regarding the risk of such decisions and there is no sensitivity information available to inform users of the most dominate life influencing factors, such as, fracture toughness, loads, or initial crack size. Due to large uncertainties involved in these key factors, a large safety factor has been selected to determine inspection intervals regardless of the results of the deterministic analyses. This large safety factor has in turn lead to smaller inspection intervals that consequently result in increased cost of operation.
Summarizing, the traditional deterministic analyses that have been used have resulted in inspection requirements that may be too conservative. A probabilistic risk assessment approach has been recommended as one way to overly conservative inspection requirements while still maintaining flight safety. In the wing carry through example, it was hoped that a probabilistic risk assessment would illustrate that there is a calculated low risk of crack growth could be used to demonstrate that grounding aircraft as they arrived at an inspection date can be safely avoided.
Mil-Std-1530C has been recently revised to include risk assessment requirements. The goals for the proposed comprehensive risk assessment process are twofold: to help determine changes to inspection and maintenance practices, and assess fleet impact for cracking found through aircraft conditional inspection (ACI), depot, or routine maintenance action. The changes to inspection and maintenances practices should result in increased inspection intervals for areas that have demonstrated minimal cracking, and a decrease in certain inspection intervals. Fleet impact assessment includes how much of the fleet may be impacted, is grounding necessary, and if the impact is fleet wide or limited. Finally, development of a guidance for Time Compliance Technical Order (TCTO) is anticipated